This invention is in the field of heat insulators and more specifically those designed in sheet form. Heat loss is typically prevented through a window by utilizing a plurality of panes of glass mounted either in a wood or metal frame. New construction allows incorporation as original equipment of such a multi-paned window. Older construction requires the addition of a window pane or storm window immediately adjacent the regular window. Such construction is quite involved and is costly to the owner. Disclosed herein is a sheet of insulation material which may be quickly mounted adjacent the window and may be removable therefrom depending upon the season. Further, the insulator disclosed herein is very inexpensive to produce, transport, store and install.